Performance of an internal combustion engine may depend on the amount of combustion air that can be delivered to the intake manifold for combustion in the engine cylinders. Atmospheric pressure is often inadequate to supply the required amount of air for efficient operation of the engine. Turbochargers are frequently utilized to increase the output of an internal combustion engine. The turbocharger may include a turbine having a turbine wheel driven by exhaust gases from the engine, and one or more compressors having compressor wheels driven by the turbine through a turbocharger shaft connected to both the turbine wheel and the compressor wheel. The spinning compressor wheel is able to force ambient air into the engine combustion chambers at a higher pressure than the engine can otherwise aspirate, resulting in what is commonly referred to as “boost pressure.” In this manner, a larger air mass and fuel mixture is achieved in the engine, which translates to greater engine output during combustion. The gain in engine output is directly proportional to the increase in air flow generated by the turbocharger boost pressure.
The boost pressure of the turbocharger may be modulated to optimize power output, for example, by varying the turbine geometry. Adjustable vanes disposed at the inlet nozzle may be used to control the flow of exhaust across the turbine wheel. The vanes can be opened incrementally wider to increase the flow cross-sectional area and permit greater gas flow across the turbine wheel, thereby causing the turbine wheel to spin at a slower speed and lowering the boost pressure. Alternatively, the vanes can be closed incrementally narrower to decrease the flow cross-sectional area and raise the boost pressure. Thus, the amount of boost pressure generated by the turbocharger can be regulated by varying the vane position.
The turbocharger having a variable turbine geometry may also provide braking for the internal combustion engine. During the braking operation, the vanes may be positioned in a restricted position in which the flow cross-sectional area is reduced, thereby increasing the exhaust pressure upstream of the turbine. The exhaust gas may flow with an increased velocity through the channels between the vanes, and the rotational velocity of the turbine wheel may increase. This increases the pressure boost of the compressor, thereby increasing the pressure of the intake air supplied to the engine. Therefore, the engine cylinders receive an increased charge pressure on the inlet side while the exhaust side experiences an elevated exhaust gas pressure. During engine operation, engine pistons may have to perform more work, for example, when there is a higher pressure in the exhaust side during the compression and exhaust strokes. Thus, increased braking can be achieved using a turbocharger with a variable turbine geometry and by setting the vanes at a restricted position.
One method of providing braking using a turbocharger with a variable turbine geometry is described in U.S. Pat. No. 6,062,025 (the '025 patent) issued to Okada et al. The '025 patent describes a brake system that includes a turbocharger and a controller for adjusting a flow cross-sectional area of a turbine in the turbocharger. By interrupting a supply of fuel to the engine, the air compressed in the combustion chamber of the engine is discharged, thereby decreasing an amount of exhaust gas output by the engine. If the exhaust flow output by the engine is small, the flow cross-sectional area of the turbine may be decreased to increase the turbine rotation speed.
Although the system of the '025 patent may permit an increase in turbine rotation speed even when the amount of exhaust gas produced by the engine decreases, additional control of the fuel injection system is necessary for interrupting the supply of fuel to the combustion chamber.
The disclosed system is directed to overcoming one or more of the problems set forth above.